1. Technical Field
The present invention relates to a class of strip transmission line circuits comprising adjustable microstrip and stripline tuners and, more particularly, to tuners which employ a pair of tuning elements in a complementary arrangement which can be used in combination to match any impedance falling within the Smith chart, each tuning element comprising a pair of adjacent, coupled or uncoupled, parallel conductive strips of equal length with one or two bridging wires, movable metallic blocks or sliding contacts bridging the gap between the strips.
2. Description of the Prior Art
Various microwave devices require the use of adjustable tuners in experimental evaluation of their performance. In the past, microstrip and stripline test fixtures were equipped with transitions to coaxial transmission lines and, therefore, coaxial-line multi-slug or multi-stub tuners could be employed. However, the large separation between the device and the tuner limited its use to frequencies less than 10 GHz. There is a need, therefore, for tuners that may be employed directly with the microstrip and stripline medium to overcome the frequency limitation of the coaxial-line tuners.
One type of tuner that is available for use with microwave transmission lines is disclosed in U.S. Pat. No. 2,757,344 issued to J. Kostriza et al on July 31, 1956, which relates to a tuner that comprises a first wide and a second narrow conductor mounted in parallel on opposite sides of a substrate. A coupling means is longitudinally movable between the adjacent conductors at a distance from the second conductor of the transmission line, and this coupling means forms, in conjunction with the first conductor of the transmission line directly adjacent the tuner, an adjustable resonant network.
The design of stripline filters and directional coupling arrangements are discussed in an article "Coupled-Strip-Transmission-Line Filters and Directional Couplers" by E. M. T. Jones et al. in IRE Transactions on Microwave Theory and Techniques, Vol. MTT-4, No. 2, April 1956 at pp 75-81. There, low-pass, band-pass, all-pass and all-stop basic filter characteristics are obtained from a pair of parallel, spaced-apart, strips either by placing open or short circuits at two of the four available terminal pairs, or by interconnecting two of the terminal pairs. The article further describes how desired performance may be achieved by cascading several of the basic filter sections.
Another design method for a class of stripline filters is discussed in the article "Synthesis of a Class of Strip-Line Filters" by H. Ozaki et al. in IRE Transactions on Circuit Theory, Vol. CT-5, No. 2, June 1958 at pp 104-109. The disclosed method relates to design on an insertion loss basis. Synthesis procedures are presented for the line type, low-pass ladder, high-pass ladder and band-pass ladder filter arrangements. The Ozaki et al. arrangements comprise line or ladder cascaded canonical filter sections with each section comprising a pair of parallel, spaced-apart, strips having either the same or different widths.
The problems remaining in the prior art is to provide a class of tuners which are capable of being formed directly on the microstrip or stripline medium and are also capable of matching any impedance falling within the Smith chart.